The present invention relates to nondestructive testing and evaluation of elongated objects such as wire cables, rods, pipes, and the like, and is concerned more particularly with a magnetic testing method and device for detecting loss in metallic cross section in the objects due to distributed or localized defects on the surface or within the object. The invention may be utilized both at the manufacturing level or in the field.
Reliable and rational techniques for assessing the condition of wire cabling, rods, pipes, and similar elongated ferromagnetic objects is not presently available. Both visual and electromagnetic inspection are known in the art, but depend extensively on the experience and intuition of the human inspector. Serious accidents, various non-scheduled equipment downtimes, and premature replacement as precautionary measures are all consequences of this state of the art, quite apart from the costs involved in the inspection process. Accordingly, it is desirable to provide a reliable technique for testing and measuring the actual strength and remaining life in the metallic objects.
One of the primary problems in the prior art magnetic inspecting devices is the bulk and weight of most instruments. Both of these factors limit the applications of the devices and reduce the resolution of the signals that are generated. Test signals are very complex and are frequently accompanied by high levels of noise due to nonhomogeneities and the coarse construction of objects under test. As a result, data interpretation often is a mixed product of both art and science. Considerable skill is required for operation of the instruments, and the instruments become rather expensive in proportion to their size.
The inspection process addresses two general types of flaws that are observed especially in wire cables. The first is a localized defect, such as a broken wire within the cabling, and the second is a distributed defect such as the loss of metallic cross section due to corrosion or abrasion. Both of these defects cause a reduction in metallic cross section and, consequently, affect cable life and strength.
There are several methods of magnetically testing elongated objects such as cables for localized or distributed defects. One of these methods is designated the main flux method and measures the amount of flux that can be carried by the cable between two longitudinally spaced stations. Since the total flux is directly related to the metallic cross sectional area of the object, measurements of the change in flux can be used to detect and measure the loss of area. U.S. Pat. No. 4,096,437 discloses a specific testing device of this type and includes Hall effect devices for measuring the amount of flux in the region of magnetic poles located at the spaced longitudinal stations along a cable. Changes in cross sectional area caused by corrosion and abrasion can be measured in absolute terms and relative movement of the cable with respect to the measuring device does not enter into the test parameters. One of the disadvantages, however, is that an extended section of the cable is inspected at any given moment. Therefore, only the average value of the metallic cross sectional area is measured with a considerable loss of resolution. Also, small flaws, such as those caused by broken wires or clusters of wires, and other localized defects cannot be detected.
Another method of testing employs a saturated magnetic field extending axially through a section of cabling under test and measures changes in leakage flux due to disruptions or breaks in the rope at the surface of the cable. Flux sensors, such as Hall effect sensors or coils, may measure the changes as the sensor and cable are moved relative to one another, and the test signals derived from the sensors may be displayed on a stripchart recorder that is driven in synchronism with the relative movement of the sensor and cable. U.S. Pat. No. 3,424,976 and U.S. Pat. No. 4,096,437 disclose specific examples of leakage flux detectors.
The advantages of leakage flux systems are that small external and internal flaws, such as broken wires, can be detected and a qualitative indication of corrosion and abrasion is also available. The disadvantages of the prior art sensors are that the reduction in cross sectional area caused by abrasion and corrosion cannot be determined quantitatively, and since the measurements are representative of changes in the leakage flux, signal amplitudes for coils are proportional to the test speeds. As a result of the latter, tachometers are generally used in connection with an automatic gain control circuit to equalize the signals for recording purposes. This adds complexity and weight to the instruments, and additionally, a certain minimum speed is generally required for a threshold signal. Because of the nonhomogeneous structure of wire cabling, test signals are very noisy, and the noise signal cannot be removed by filtering because the differences in the levels of the noise and flaw signals are very small. Still further, because of the requirement for movement between the magnetic device and the cable, the process cannot be carried out at the ends of a cable which is permanently secured in place.
Accordingly, it is a general object of the present invention to provide a method and apparatus for quantitatively determining the loss of metallic cross section caused by corrosion, abrasion, and other factors, and to also obtain at least a qualitative measurement of localized defects without the disadvantages mentioned above.